Photopolymerizable resist materials are known to form a resist layer on a substrate, such as a printed circuit board, to allow subsequent processing of the substrate such as selective etching, electroplating or treatment with solder. The use of a non-photopolymerizable secondary layer in combination with photosensitive materials, such as a photopolymerizable resist, is known to enhance particular properties of the photoresist such as surface hardness, phototool release and photosensitivity.
The use of a solvent developable multiple layer photoresist for use in the etching of printed circuit detail is known. U.S. Pat. No. 4,349,620 describes such a photoresist that has improved adhesion, without sacrifice in development time, and with reduced film thickness. The use of a two layer photoresist system is disclosed in U.S. Pat. No. 4,352,870. Improved resolution is achieved through the resist layer closer to the substrate having a greater photosensitivity.
Lamination of a photosensitive material onto a substrate employing an intermediate liquid layer is taught in several U.S. Pat. Nos. using various liquids and application techniques. U.S. Pat. No. 3,629,036 discloses application of a liquid adhering agent, preferably a solvent for a resist containing a small amount of dissolved resist, to a surface of a substrate followed by application of a photosensitive resist film. U.S. Pat. No. 4,069,076 discloses a process for applying a photoresist film to a preimaged pattern relief substrate after flooding the substrate with a solvent or a nonsolvent swelling agent to reduce air entrapment and improve resist conformation. U.S. Pat. No. 4,293,635 discloses application of a photosensitive composition containing an amphoteric interpolymer, for improved cold flow, to a copper surface wetted with a liquid such as ethanol-water solution. U.S. Pat. No. 4,698,294 discloses a process for lamination of a dry film with a lamination liquid, which is substantially all monomer and non-photosensitive, such that adequate conformation and adhesion of the dry film to the circuit board is obtained with no air entrapment.
U.S. Pat. No. 4,506,004 discloses various embodiments of obtaining printed circuit boards more cost effectively by means of a two layer composite coating. In one embodiment an adhesive photopolymer layer is applied to a printed wiring board in a liquid state displacing air from the printed wiring board surface. A dry film solder mask is temporarily adhered to the underside of a screen printing frame and applied onto the liquid layer prior to processing of the photopolymer layer. In another of the embodiments of the patent nonphotoimaging epoxy layers can be employed.
In the use of permanent coatings for printed circuit boards, a combination of storage stability and acceptable end-use properties such as high temperature (solder) resistance, chemical inertness, high electrical resistance, low electromigration and good adhesion to raised copper metal reliefs is difficult to achieve, particularly with the use of liquids alone or dry films alone. Photoimageable liquid permanent coatings are known to suffer from pin-holes and skips as well as insufficient mask thickness at the corner of raised copper metal reliefs. Dry film photoimageable permanent coatings will typically alleviate the drawbacks of liquids, but are more costly and do not conform as well to the raised copper reliefs as liquids. It would be useful to have a permanent coating with an acceptable balance of the required storage, application and performance properties and particularly useful for the coating to be aqueous processable.